Behavior recognition system

ABSTRACT

A system for recognizing various human and creature motion gaits and behaviors is presented. These behaviors are defined as combinations of “gestures” identified on various parts of a body in motion. For example, the leg gestures generated when a person runs are different than when a person walks. The system described here can identify such differences and categorize these behaviors. Gestures, as previously defined, are motions generated by humans, animals, or machines. Multiple gestures on a body (or bodies) are recognized simultaneously and used in determining behaviors. If multiple bodies are tracked by the system, then overall formations and behaviors (such as military goals) can be determined.

REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/850,896, filed Mar. 26, 2013, which is a continuation of U.S. patentapplication Ser. No. 11/412,252, filed Apr. 25, 2006, now U.S. Pat. No.8,407,625, which is a continuation of U.S. patent application Ser. No.09/540,461, filed Mar. 31, 2000, now U.S. Pat. No. 7,036,094, whichclaims priority of U.S. provisional application Ser. No. 60/127,510filed Apr. 2, 1999.

U.S. patent application Ser. No. 09/540,461 is a continuation-in-part ofU.S. patent application Ser. No. 09/371,460, filed Aug. 10, 1999, nowU.S. Pat. No. 6,681,031, which claims priority from U.S. ProvisionalPatent Application Ser. No. 60/096,126, filed Aug. 10, 1998. The entirecontent of each application and patent are incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates generally to automated image recognition and, inparticular, to computer-based behavior recognition methods andapparatus.

BACKGROUND OF THE INVENTION

Differentiating between normal human activity and suspicious behavior isa difficult task, whether performed by a sensing device or a humanobserver. A human observer would find such a task tedious and costly toperform in money and time. Fortunately, a sensing system is not botheredby ‘tedious’ tasks. Such a system could be implemented to prune outobviously normal behavior, and tag human activities which could besuspicious and would therefore need more attention by a human operator.

However, such “behavior recognition” systems have not been developed dueto the difficulty of identifying and classifying such motions. Consideran urban environment as illustrated in FIG. 1. In such an environment,there are many moving objects and people, most of which are not activelyengaged in criminal or destructive behavior. All of these activitieswould have to be viewed and identified before they could be removed frompotential further consideration by a human.

SUMMARY OF THE INVENTION

The present invention is directed to the automated classification andidentification of human activities. The requisite heuristics involverecognition of information-bearing features in the environment, and thedetermination of how those features relate to each other over time. Theapproach resides in a behavior recognition sensor system whichidentifies simultaneously created gestures using a gesture recognitionsystem. The underlying gesture recognition system performs the task ofdetermining the behavior (state) of objects in motion.

According to the invention, general kinematic relationships for variousbodies (human, animal, robotic) are modeled. Next, specific linkcombinations for each are parameterized and modeled. This enables thesystem to recognize such motions as the various human motion gaits (ifthe links are legs), the throwing of an object (if the links are arms),or any other type of motion and orientation. A whole-body (human,vehicle, or other) link representation and dynamic model is thendeveloped.

The state of an object (a human or a device) can take three forms.

-   -   The object is static (motionless) and cannot be easily moved by        a human agent. Such objects would include walls, filing        cabinets, telephone poles, and the like. These are identified        and localized with respect to the vision system;    -   The object is static but can be manipulated by a human agent.        Such objects would include weapons, chairs, etc. These are        localized with respect to the vision system, tagged, and        identified and evaluated with regard to their potential use by        human agents; and    -   The object is in motion. Such objects include self-mobile        objects such as humans and vehicles, as well as objects that are        carried by humans or vehicles, such as guns or briefcases.

Whenever an object is identified, the system preferably first determineswhich state the object is in based upon gesture recognition. The term“gesture” not only refers to the (dynamic) motion of an object, but alsoto the state of an object that is not moving (static). For example, agun being pointed by a human should definitely be classified as agesture. Therefore, determining behaviors also means identifyingcombinations of static and dynamic gestures.

An object is composed of many connected non-deformable links. Forexample, a person is made up of a torso (one link), a head (one link),two arms (each with two links), two hands (one link each, not countingthe fingers), two legs (each with two links), and two feet (one linkeach). Each link of an object has its own gesture motion, which, whenexamined in relation to the other links in the object, can be used todetermine the overall state of the system. For example, when justexamining the gestures created by the foot and knee joint of a human,one can see that the motions of those features are different dependingon whether a person is walking or running. Even when not in motion, therelationship of those non-deformable links gives rise to informationabout the object's state. A person who is standing still will have adifferent kinematic link relationship to one who is sitting, or lyingdown.

Such gross human torso features can be readily identified (using a modelthat matches general body features) with the vision system described inco-owned U.S. Pat. No. 6,681,031. Building on this technology, thegesture recognition module of this invention determines if a dynamicmotion is occurring, and uses that information with kinematic linkrelationships, develop a hypothesis about the overall state of theobject or objects in the field of view.

In a sense, then, the invention provides an automatic method forclassifying and categorizing such dynamic motions and gaits. Such staterecognition is not limited to humans (and other animals), however. Avehicle's state can also be determined by examining the various movingparts, such as the body and the tire motions. Even unknown devices (suchas mobile robots) can be classified by examining their motion andbehavioral characteristics.

Although subject visualization is preferred, alternative approaches togait/behavior recognition may be employed according to the invention,including electro-mechanical methods of tracking the body to identifyhuman motion. This includes using:

-   -   electrogoniometers and electrogoniometric systems (EGM)    -   passive reflective and actively illuminated markers to calculate        positions and velocities, i.e. raw data.    -   force data gathered from a force plate or force dynamometer.        Used to calculate internal joint moments causing motion.    -   strain gage or piezoelectric transducers to measure ground        reaction forces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of potential behavior recognition system uses;

FIG. 2 is a drawing of a gesture recognition system according to theinvention;

FIG. 3 is a gesture recognition system flow chart;

FIG. 4 is a signal flow diagram of a gesture recognition systemaccording to the invention;

FIG. 5 is a drawing which shows example gestures in two dimensions;

FIG. 6 shows three example gestures;

FIG. 7 is an example of a 24-gesture lexicon according to the invention;

FIG. 8 depicts a Slow-Down gesture;

FIG. 9 depicts a Move gesture;

FIG. 10 depicts an Attention gesture;

FIG. 11 depicts a Stop gesture;

FIG. 12 shows Right/Left Turn gestures;

FIG. 13 shows an “Okay” gesture;

FIG. 14 shows a Freeze gesture;

FIG. 15 provides three plots of a human created one dimensional X-Lineoscillating motion;

FIG. 16 shows possible lines associated with x(t,p)=p0+p1t and theirequivalent representation in the p-parameter space;

FIG. 17 illustrates parameter fitting wherein a rule is used for q tobring the error to zero;

FIG. 18 plots different (xi,yi) data points resulting in a differentbest fitting q line;

FIG. 19 depicts a recursive linear least squares method for updating qwith subsequent (xi,yi) data points;

FIG. 20 illustrates an algorithm for determining a specific gesturemodel according to the invention;

FIG. 21 is an exaggerated representation of a residual errormeasurement;

FIG. 22 is a plot which shows worst case residual ratios for eachgesture model, wherein the lower the ratio, the better the model;

FIG. 23 illustrates how two perpendicular oscillatory line motions maybe combined into a circular gesture;

FIG. 24 shows how a bounding box may be placed around a hand associatedwith a gesture;

FIG. 25 provides descriptions from the bounding box of FIG. 24;

FIG. 26 shows example gestures;

FIG. 27 is a schematic of hand-tracking system hardware according to theinvention;

FIG. 28 is a flowchart of a color tracking system (CTS) according to theinvention;

FIG. 29 depicts a preferred graphical user interface of the CTS;

FIG. 30 illustrates the application of target center from differenceimage techniques;

FIG. 31 illustrates a color matching technique;

FIG. 32 is a representation of an identification module;

FIG. 33 is a simplified diagram of a dynamic gesture prediction moduleaccording to the invention; and

FIG. 34 is a simplified diagram of a behavior recognition moduleaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The core of the behavior recognition system is a set of dynamic andstatic gesture recognition modules. This section details the overallgesture recognition system used by the behavior recognition system.

The Gesture Recognition System

FIG. 2 presents a system overview of a gesture controlled self servicemachine system. FIG. 3 shows a flow chart representation of how a visionsystem is views the gesture created, with the image data sent to thegesture recognition module, translated into a response, and then used tocontrol a SSM, including the display of data, a virtual environment, anddevices. The gesture recognition system takes the feature positions ofthe moving body parts (two or three dimensional space coordinates, plusa time stamp) as the input as quickly as vision system can output thedata and outputs what gesture (if any) was recognized, again at the samerate as the vision system outputs data.

The specific components of the gesture recognition system is detailed inFIG. 4, and is composed of five modules:

G: Gesture Generation

S: Sensing (vision)

I: Identification Module

T: Transformation

R: Response

At a high level, the flow of the system is as follows. Within the fieldof view of one or more standard video cameras, a gesture is made by aperson or device. During the gesture making process, a video capturecard is capturing images, producing image data along with timinginformation. As the image data is produced, they are run through afeature tracking algorithm which outputs position and time information.This position information is processed by static and dynamic gesturerecognition algorithms. When the gesture is recognized, a commandmessage corresponding to that gesture type is sent to the device to becontrolled, which then performs and appropriate response. The fivemodules are detailed below.

Gesture Creator

In the Gesture Creator module, a human or devices creates a spatialmotion to be recognized by the sensor module. If one camera is used,then the motion generated is two dimensional and parallel to the imageplane of the monocular vision system. For three dimensional tracking (asis also done with this system), stereo vision using two or more camerasare used.

This gesture recognition system is designed to recognize consistent yetnon-perfect motion gestures and non-moving static gestures. Therefore, ahuman can create such gestures, as well as an actuated mechanism whichcould repeatedly create perfect gestures. Human gestures are moredifficult to recognize due to the wide range of motions that humansrecognize as the same gesture. We designed our gesture recognitionsystem to recognize simple Lissagous gesture motions (repeating circlesand lines), repeated complex motions (such as “come here” and “go awayquickly” back and forth hand motions which we define as “skew”gestures), and static hand symbols (such as “thumbs-up”).

With regards to human generated gestures used for communication ordevice control, we chose gestures to be identified based on thefollowing:

Humans should be able to make the gestures easily.

The gestures should be easily represented mathematically.

The lexicon should match useful gestures found in real worldenvironments.

For the dynamic (circular and skew) gestures, these consist ofone-dimensional oscillations, performed simultaneously in two or threedimensions. A circle is such a motion, created by combining repeatingmotions in two dimensions that have the same magnitude and frequency ofoscillation, but with the individual motions ninety degrees out ofphase. A “diagonal” line is another such motion. We have defined threedistinct circular gestures in terms of their frequency rates: slow,medium, and fast. An example set of such gestures is shown in FIG. 5.These gestures can also be performed in three dimensions, and such morecomplex motions can be identified by this system.

The dynamic gestures are represented by a second order equation, one foreach axis:

-   -   {dot over (x)}₁=x₂    -   {dot over (x)}₂=θ₁x₁+θ₂

More complex second order models are used to recognize more complexgestures (discussed later). This gesture model has no “size” parameter.θ₁ is a frequency measure, and θ₂ is a drift component. The gestureswere named “large”, “small”, “fast”, and “slow” due to the human motionsused to determine the parameters (see FIG. 6). A fast small circle isused to represent a fast oscillation because humans cannot make fastoscillations using large circles.

For example, a total of 24 gestures are possible when the following aredistinct gestures: clockwise and counter-clockwise circles, diagonallines, one dimensional lines, and small and large circles and lines.Geometric constraints are required to expand the lexicon, becausedifferent gestures can result in the same parameters. FIG. 5 showsmotions that would cause an identifier to produce the same frequencymeasure and drift components as it would produce when identifying a slowlarge circle. When x and y oscillating motions are 90 degrees out ofphase, a clockwise circle is produced. Motions that are 270 degrees outof phase result in a counter clockwise circle. In phase motions producea line with a positive slope. When the motions are 180 degrees out ofphase, a line with a negative slope is produced. We can createadditional gestures from the fast small circle in the same manner.

As with the previous gestures, additional gestures can be created fromthese two gestures by varying the phase relationships. FIG. 7 shows arepresentation of the 24 gestures in possible lexicon. Even moregestures are possible when the third dimension is used.

Phase relationships are determined as follows. During the gesture, thex's and y's (and z's, if the system is set up for three dimensions)minimum and maximum image plane positions are computed. If the x and ymotions are out of phase, as in a circle, then when x or y is minimum ormaximum, the other axis's velocity is large. The direction(clockwiseness in two dimensions) of the motion is determined by lookingat the sign of this velocity component. Similarly, if the x and y motionare in phase, then at these extremum points both velocities are small.

Example dynamic gestures used for real world situations are taken from a1987 Army Training Manual. A “Slow Down” gesture is a small x-linecreated to one side of the body (FIG. 8, left side). A “Day Move”gesture is a counterclockwise large slow circle (FIG. 9, left side). The“Attention” gesture is a large y-line overhead motion (FIG. 10). Thesethree gestures are representative of the motion gestures used throughoutthe Army manual.

Static gestures are represented as geometric templates. Four gesturesare shown and are representative of the static gestures which can berepresented and identified by this gesture recognition system.Additionally, language gestures, such as American Sign Languagegestures, can also be recognized.

The example static gestures are:

-   -   Halt—stop hand above head (FIG. 11—left side of figure).    -   Left and Right turn—fingers together, palm out, facing left or        right (FIG. 12—left side of figure).    -   Message Acknowledge (OK)—thumb up (FIG. 13).    -   Freeze—first at head level (FIG. 14).

Identifying Moving Gestures Represented as a Dynamic System

The gesture recognition system identifies a moving gesture by itsdynamics—that is, the structure of its positions in space over time. Thesystem translates the motion information into parameters which are usedto develop commands for controlling data outputs and actuatedmechanisms. For example, the speed at which a person waves a robot awaymight directly affect a robot arm's velocity or a mobile robot's speed.In order for recognition to occur, a representation for human gesturesis required, from which a computational method for determining andrecognizing specific gestures can be derived.

Although we make these gestures in two and three dimensions, theexplanation now detailed is described simply dimension as a basic onedimensional gesture as a simple example to clarify the distinctionbetween the “shape” and the “dynamics” of a gesture. The techniques foridentifying this basic gesture will be used to identify similaroscillatory motions occurring in two and three dimensions.

First, a dynamic system gesture representation is determined, both themodel for representing the oscillatory gestures and parameterdetermination scheme was developed. For this system a Linear LeastSquares method was an on-line computationally efficient technique whichallowed us to use a linear-in-parameters gesture model.

The representative planar gesture used throughout this section toexemplify our method consists of a family of oscillating motions whichform a (roughly) horizontal line segment (“x-line motion”). As discussedearlier, a human is incapable of reliably generating a perfectsinusoidal motion. FIG. 15 illustrates the imperfections of a humancreated x-line motion viewed in three plots. The plots represent theposition of the gesture over time, x(t). Viewing position with respectto time in contrast to position and velocity over time provides insightinto how we propose to represent gestures. Plot A shows the planarmotion in x-position and y-position coordinates, with the gesture'smotion constrained to the x-axis. Thus, the “shape” of the motionconveys relatively little information. Plot B shows the same gesture inx-position plotted against time, emphasizing the oscillatory behavior wewish to capture. Plot C represents the record of x-velocity plottedagainst x-position over time. We will find it most convenient torepresent this motion as it evolves over time in this position versusvelocity space, which is called the “phase plane”. Of course, when ahuman creates a gesture, the resulting motion does not translate intothe perfect sinusoid of plot B or a perfect circle of plot C. Instead,there is a natural range of variation that we would nevertheless like toassociate with the same gesture. This association we find most naturallyachievable in phase space.

For this dynamic gesture recognition module, a computationally effectivemathematical representation for the gesture plotted in FIG. 15 isrequired. A general representation for time functions might take theform

x(t)=?

where “?” would be replaced with some structure based on measurablefeatures which are used to classify the gesture. Of course, there are aninfinite number of possible measurable features.

We can make the number of classifications (the “feature space”dimension) finite by restricting the form of the representations.Instead of representing gestures as x(t), the representation might beconstrained through the use of a parameter vector, resulting in x(t,p).The feature space dimension is then equivalent to the number ofparameters we store. For example, when:

x(t,p)=p ₀ +p ₁ t, t>0,

the only possible gestures that we can represent are lines described bythe two parameters slope, p₁, and intercept p₀ (see FIG. 16).

Even with a finite dimensional representation, each unique motion isrepresented by its own distinct parameters. However, our intuition abouthuman gestures tells us that certain distinct motions should have thesame classification. Consider the x-line oscillating gesture discussedearlier. Whether the gesture starts at the left side of the line or theright side (for example, x(0)=−1 or x(0)=+1), the resulting motionswould still be identified by a human as the same gesture. Therefore,another type of representation seems desirable.

Since a human hand forms a gesture, we could imagine a representation interms of the force exerted by the person's arm muscles. Alternatively,we might imagine representing the gesture as a function of the nerveimpulses that travel from the brain to the arm's muscles. However, quiteclearly, most of the countless types of such “internal” representationsare presently impossible to quantify in any useful manner. Four hundredyears ago, Newton developed a parsimonious representation of physicalmotions based on their dynamic properties,

{dot over (x)}(t)=f(x)

A dynamic system is a mathematical model describing the evolution of allpossible states in some state space as a function of time [Hirsch 74][Arnold 78]. The set of all possible states is a state space. Given aninitial state, the set of all subsequent states as it evolves over timeis a “trajectory” or “motion”. For any initial condition, the futureevolution of the states in a trajectory remains within that trajectory(the trajectory is an invariant set). Thus, all that is required todescribe a particular spatial motion is the differential equationrepresentation and its initial conditions. We use a deterministicrepresentation, as opposed to a stochastic one, because we believe theseoscillatory motions are best represented by sine waves or a sum ofexponentials as opposed to characteristics based on statisticalproperties. As with the geometric representation, there are an infinitenumber of gesture classifications of the form {dot over (x)}(t)=f(x).However, as before, we can choose a vector of tunable parameters to makethe number of gesture classifications finite. Such representation hasthe form:

{dot over (x)}(t)=f(x,σ)

where θ represents the tunable parameters. Fixing the value of θ in agiven representation yields a unique set of motions, with differentinitial conditions, described by {dot over (x)}(t)=f(x,θ). Motivated bythe way humans interpret gestures, we associate an entire set of motionswith one specific gesture. Thus, choosing different values of θ in agiven representation results in a “family” of trajectories sets—a“gesture family.” For example, consider a oscillatory line gesture, themotion of which is constrained to the x-axis. This gesture can berepresented in the following two dimensional state space:

{dot over (X)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁

where x₁ represents the position of the gesture, x₂ is its velocity, andθ₁ is a specified negative parameter. For any constant θ>0, alltrajectories satisfy −θ₁x₁ ²+x₂ ²=const as can be seen by directdifferentiation.

We conceive of a specific gesture as a family of sets of trajectories.Referring to the gesture in FIG. 15, a human can start the gesture atany point (initial condition) in its trajectory, and the gesture shouldstill be identified as the same oscillating line.

We will now represent a given family of gestures (family of sets oftrajectories) by a mathematical model which contains a finite number oftunable parameters. A mathematical model described by differentialequations, as above, allows the development of a computational schemethat will determine which parameters, the values of θ_(i)'s, correspondto a specific gesture. The set of all valid parameters is the parameterspace. The parameter space defines the family of gestures which can berepresented by the model. In order to categorize a finite number ofgestures in this family and to permit further variability in the exactmotions associated with a particular gesture within this family, wepartition the parameter space into a finite number of cells—the“lexicon”—and associate all the parameter values in the same cell withone gesture.

We will “invent” certain differential equations, composed of statevariables and parameters, which intuition suggests may represent humangestures. Such differential equation models can be divided into twotypes: non-linear-in-parameters (NLIP) and linear-in-parameters (LIP).The two models can be further subdivided into linear-in-state (LIS) andnon-linear-in-state (NLIS). It is advantageous to use a NLIP (with NLIS)model because it covers, by definition, a much broader range of systemsthan an LIP model. However, for reasons to be discussed below, we findit expedient to use a LIP model for our gesture representation. We havechosen to represent planar oscillatory gestures as a second order systemwith the intuition that a model based on the acceleration behavior(physical dynamics) of a system is sufficient to characterize theoscillatory gestures in which we are interested. This system's statesare position and velocity. However, the vision system we use to sensegestures yields only position information. Since velocity is notdirectly measured, then either the parameter identification method couldbe combined with a technique for observing the velocity, or the velocitycould be determined through position differences. In this section weshow techniques for determining gesture parameters both when thevelocity state is observed, and when it is obtained through positiondifferences. By examining the utility of each technique, we develop anappropriate form of the gesture model and parameter identificationmethod.

A difficulty with using human created gestures is that the underlyingtrue physical model is unknown. Also, because people cannot preciselyrecreate even a simple circular gesture, multiple sets of parameterscould represent the same gesture. Simulations are used both to determinea viable gesture model and to determine if it is possible to discoverappropriate parameters for each gesture despite variations in motion.

We had chosen to represent motion gestures using dynamic systems. Ournext task was to determined a model and a method for computing themodel's parameters such that the model's parameters will best match anobserved gesture motion. FIG. 17 illustrates how the gesture's positionis used as an input, with {circumflex over (θ)} representing the unknownparameter values that we wish to match with the “true” parameter values,θ. If these values match, then the error between the true states {dotover (x)} and the observed states {dot over ({circumflex over (x)})}will go to zero.

Our choice of a model and parameter determination scheme was based on anexploration of the following issues:

-   -   Off-line batch techniques versus on-line sequential techniques.        We desire our gesture recognition system to identify gestures as        they are generated, which requires an on-line technique. Also,        the measure of how well a motion matches a gesture's parameters        needs to be updated “on-line”.    -   State availability. Using a vision system to sense gestures        results in image plane position information. However, we are        using a second order system to describe gestures. Therefore, we        need both positions and velocities for our residual error        measurements (see below). Velocity can be obtained through the        use of an estimator or by taking a difference of position        measurements. Unfortunately, using differences adds noise to the        data, which could make parameter identification difficult.    -   Data order dependent versus independent (for on-line        techniques). Certain on-line techniques will produce different        parameter values based on the order the gesture data is        presented. Because we define a gesture as a family of        trajectories, with each trajectory in the same family equally        valid, our method should be data order independent. In        particular, different excursions through the same data set        should result in the same parameters at the end of the data        acquisition phase.    -   Linear versus Non-Linear. A model is a combination of linear and        non-linear states and parameters. Although perfect (non human        created) circular oscillatory motions can be described by a        linear-in-parameters and linear-in-states model, a human created        gesture may require a more complex model. Furthermore, our        system can recognize more complex oscillatory motions.        Therefore, a method for identifying parameters in a richer        non-linear model is needed, because non-linear models can        represent a much broader range of motions.

We chose our gesture model and parameter determination scheme asfollows. First, we decided to abandon off-line batch techniques in favorof on-line ones for reasons already discussed above. The on-line methodneeds to be chosen carefully, because there are relatively few caseswhere it can be guaranteed that the estimated parameters will beequivalent to those resulting from off-line techniques applied to theentire data set as a whole.

Next, in an attempt to use only position data, we examined aSeries-Parallel Observer, which provides an estimate of the otherunknown state for purely LIS and LIP systems. We abandoned this observerbecause it cannot adequately estimate parameters of non-perfect humangestures. Specifically, we could not extend the method to NLIS systems.

An on-line gradient descent method was examined, but for presentlyavailable methods applicable to NLIP systems, there is no guarantee thatthe parameters will converge towards their optimal values. Also, theparameters computed via this method are dependent on the order the datais presented.

A Linear Least Squares method (LLS) was examined next, which makes useof all the data independent of ordering. The resulting recursive LLStechnique work for NLIP models, and, therefore, allow us to examine moreflexible and useful gesture models. [See Cohen 96 for a detaileddiscussion of the reasons for the above decisions.]

The Recursive Linear Least Squares incrementally incorporates new datafor determining the parameters which will best fit a set of data pointsto a given linear model. The recursive LLS method uses a tuning rule forupdating the parameter vector θ without inverting a matrix, creating amore computationally efficient LLS algorithm. A tuning rule is required,because each block of data will result in a different set of parameters,as illustrated in FIG. 18. The separate graphs show that each pair of(x_(i),y_(i)) data points results in a different best fitting θ line.

A method of incrementally updating the parameter θ is described below.For full details, see [Kumar 86]. The concept is illustrated in FIG. 19.After the first two data points determine the best fit line, eachadditional data point slightly adjusts the line to a new best fit. Eachnew data point will shift the line less and less due to the weightingauxiliary equation in the recursive LLS method. The formulation belowdescribes how the weighting function operates.

The recursive (incremental) Linear Least Squares tuning method proceedsas follows. The tuning rule has the form:

θ_(m+1) =g(x _(m) ,{dot over (x)} _(m),θ_(m))

Suppose we have the output data {dot over (x)} and state data x up totime in, and from this data we have already determined the bestparameters θ for the set. From [Cohen 96] we know that at the next timestep, with {dot over (x)}_(m+1) and x_(m+1):

$\theta_{m + 1} = {\left( {\sum\limits_{k = 1}^{m + 1}\; {f_{k}^{T}f_{k}}} \right)^{T}{\sum\limits_{k = 1}^{m + 1}\; {f_{k}^{T}{\overset{.}{x}}_{k}}}}$

Define

$R_{m + 1} = {\sum\limits_{k = 0}^{m + 1}\; {f_{k}^{T}{f_{k}.}}}$

Then:

$\begin{matrix}{R_{m} = {\left( {\sum\limits_{k = 0}^{m - 1}\; {f_{k}^{T}f_{k}}} \right)^{- 1} + f_{m}^{T}}} \\{= {R_{m - 1} + {f_{m}^{T}f_{m}}}}\end{matrix}f_{m}$

which implies:

R _(m−1) =R _(m) −f _(m) ^(T) f _(m)

Therefore:

$\begin{matrix}{\theta_{m + 1} = {R_{m + 1}^{- 1}{\sum\limits_{k = 1}^{m + 1}\; {f_{k}^{T}{\overset{.}{x}}_{k}}}}} \\{= {R_{m + 1}^{- 1}\left( {{\sum\limits_{k = 1}^{m}\; {f_{k}^{T}{\overset{.}{x}}_{k}}} + {f_{m + 1}^{T}{\overset{.}{x}}_{m + 1}}} \right)}} \\{= {R_{m + 1}^{- 1}\left( {{\sum\limits_{k = 1}^{m}\; {f_{k}^{T}f_{k}\theta_{m}}} + {f_{m + 1}^{T}{\overset{.}{x}}_{m + 1}}} \right)}} \\{= {R_{m + 1}^{- 1}\left( {{R_{m}\theta_{m}} + {f_{m + 1}^{T}{\overset{.}{x}}_{m + 1}}} \right)}} \\{= {R_{m + 1}^{- 1}\left( {{\left( {R_{m + 1} - {f_{m + 1}^{T}f_{m + 1}}} \right)\theta_{m}} + {f_{m + 1}^{T}{\overset{.}{x}}_{m + 1}}} \right)}} \\{= {R_{m + 1}^{- 1}\left( {{R_{m + 1}\theta_{m}} - {f_{m + 1}^{T}f_{m + 1}\theta_{m}} + {f_{m + 1}^{T}{\overset{.}{x}}_{m + 1}}} \right)}} \\{= {\theta_{m} - {R_{m + 1}^{- 1}{f_{m + 1}^{T}\left( {{f_{m + 1}^{T}{\overset{.}{x}}_{m + 1}} - {f_{m + 1}\theta_{m}}} \right)}}}}\end{matrix}$

This is an update law for the R_(m+1) and θ_(m+1) terms. We still haveto find the inverse of R_(m+1) at each time step. Fortunately, thematrix inversion lemma yields:

(R _(m) ,f _(m) ^(T) f _(m))⁻¹ =R _(m) ⁻¹ −R _(m) ⁻¹ f _(m) ^(T)(f _(m)R _(m) ⁻¹ f _(m) ^(T)+1)⁻¹ f _(m) R _(m) ⁻¹

Therefore:

$\begin{matrix}{R_{m + 1}^{- 1} = \left( {R_{m} + {f_{m}^{T}f_{m}}} \right)^{- 1}} \\{= {R_{m}^{- 1} - {R_{m}^{- 1}{f_{m}^{T}\left( {{f_{m}R_{m}^{- 1}f_{m}^{T}} + 1} \right)}^{- 1}f_{m}R_{m}^{- 1}}}}\end{matrix}$

The above equation is a recursive formula for R_(m+1) ⁻¹ that is notbased on taking the inverse of a matrix. The initial value of R₀ ischosen as the identity matrix. See [Cohen 96] for a more formaldiscussion. If more importance is attached to recent data than to datareceived in the remote past, then we can choose θ_(m) to minimize:

$\sum\limits_{k = 0}^{m}\; {\lambda^{m - k}\left( {{\overset{.}{x}}_{k} - {f_{k}^{T}f_{k}}} \right)}$

where λ is termed the forgetting factor and is chosen with 0<λ<1. Thisresults in:

$\theta_{m + 1} = {\theta_{m} + {R_{m + 1}^{- 1}{f_{m + 1}^{T}\left( {{\overset{.}{x}}_{m + 1} - {f_{m + 1}\theta_{m}}} \right)}}}$$R_{m + 1}^{- 1} = {{\frac{1}{\lambda}R_{m}^{- 1}} - {\frac{1}{\lambda}R_{m}^{- 1}{f_{m}^{T}\left( {{f_{m}R_{m}^{- 1}f_{m}^{T}} + \lambda} \right)}^{- 1}f_{m}R_{m}^{- 1}}}$

The above recursive equation is the identifier in our gesturerecognition system. This identifier allows us to represent gesturesusing a NLIP model, with the parameters identified using an on-linecomputationally efficient data order independent technique. We nowdetermine the specific model used to represent oscillatory motiongestures.

Given that we modeled gestures using an LIP/NLIS representation, thefollowing algorithm was used to determine the appropriate model (seeFIG. 20).

For the first step, we created phase-plane plots of the gestures to bemodeled, as illustrated in the last plot in FIG. 15. A term in adifferential equation model was composed of a parameter associated withcombinations of multiplied state variables of various powers, that is,of the form θ_(i)x₁ ^(j)x₂ ^(k). An example model (of a one dimensionalmotion is):

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂

Intuition was used to “guess” appropriate models that would best matchthe phase plane motions. Because we believed an acceleration model willsufficiently characterize the gestures in which we are interested, the{dot over (x)}₂ equation is the one modified with additional terms andparameters. For each model, the specific parameters for each gesture inthe lexicon were computed using the LLS method (as discussed earlier).

The models were tested in simulation by measuring how well each tunedparameter model can predict the future states of its associated gesture(i.e. computing a total residual error). The model which bestdiscriminates between gestures was chosen.

If none of the models can clearly discriminate between differentgestures in a lexicon, then new models are required. The heuristic weused was to add or delete specific terms, and determine if there was asignificant change (good or bad) in the model's ability to discriminategestures. Adding two specific terms to the above equation, that is,using the new model

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂ x ₂+θ₃ x ₂ x ₁ ²+θ₄

results in a model that is better able to discriminate between gestures.

The results of this process of modeling oscillating circles and linesare detailed in the remaining parts of this section. This process willalso be used in the Phase II effort to determine an appropriate model toclassify certain non-linear gestures.

A variety of linear-in-parameter models for good circle and line gesturerepresentations were tested. As before, each model represented only onedimension of motion, which was expanded to two or three for actuallygesture recognition (i.e. an oscillating circle or line is formed whentwo or three of these decoupled models are present, one for each planarmotion dimension). Again, x₁ is the position state, and x₂ is thevelocity state. Five of these models are shown below. The determinationof such models illustrates how a new (and more comprehensive model)could be determined when required for more complex dynamic motions.

To use the models described here on a digital computer, a fourth-orderRunge-Kutta integration method was used [Press 88]. Simulations showedthat a sampling rate of 10 Hz is sufficiently small to allow the use ofthis method.

The linear with offset component model is the most basic second orderlinear system. The offset component allows the model to representgestures that are offset from the center of the image plane. It containstwo parameters and is of the form:

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂

The Van der Pol equation is a slightly non-linear system, containingthree parameters. The θ₂ and θ₃ parameters are attached to dampingterms. This system is of the form:

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂ x ₂+θ₃ x ₂ x ₁ ²

An offset component is added to the Van der Pol equation in this system.This system has four parameters and is of the form:

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂ x ₂+θ₃ x ₂ x ₁ ²+θ₄

A more non-linear system than the Van der Pol equations, the HigherOrder Terms system contains additional spring-like components. Thissystem has six parameters and is of the form:

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂ x ₁ ²+θ₃ x ₁ ³+θ₄ x ₂+θ₅ x ₂ x ₁ ²+θ₆

The Velocity Damping Terms system has additional damping terms. Itcontains eight parameters and is of the form:

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂ x ₁ ²+θ₃ x ₁ ³+θ₄ x ₂+θ₅ x ₂ x ₁ ²+θ₆ x ₂³+θ₇ x ₁ ² x ₂ ³+θ₈

The use of simulations to determine the best gesture model forrepresenting oscillating circles and lines is now detailed. We firstdetails the residual measure calculation. Next the use of the residualmeasure to determine the best gesture model is described.

A predictor bin is composed of a model with parameters tuned torepresent a specific gesture. The role of a bin is to determine agesture's future position and velocity based on its current state. Tomeasure the accuracy of the bin's prediction, we compared it to the nextposition and velocity of the gesture. The difference between the bin'sprediction and the next gesture state is called the residual error. Abin predicting the future state of a gesture it represents will have asmaller residual error than a bin predicting the future state of agesture it does not represent.

The computation for the residual error is based on equation:

{dot over (x)} _(k) =F _(k) ^(T)θ

Recall that f(x) is a two dimensional vector representing the gesture'sposition and velocity. Therefore {dot over (x)}_(k) is the gesture'svelocity and acceleration at sample k. We compute {dot over (x)}_(k)from the gesture's current and previous position and velocity. Theparameter vector {circumflex over (θ)} is used to seed the predictorbin. Then:

{dot over ({circumflex over (x)})}_(k) =f _(k) ^(T){circumflex over (θ)}

The residual error is then defined as the normalized difference betweenthe actual value of {dot over (x)}_(k) and the calculated value of {dotover ({circumflex over (x)})}_(k):

${res\_ err} = \frac{{{\overset{.}{x}}_{k} - {\overset{.}{\hat{x}}}_{k}}}{{\overset{.}{x}}_{k}}$

FIG. 21 illustrates this concept. Consider the gesture at a givenvelocity and acceleration, sample k. At sample k+1, the predictions fromeach bin and the actual velocity and acceleration values are shown. Thedifference between a bin's predicted values and the gesture's actualvalues (according to equation above) is the residual error for thatparticular bin.

The total residual error is the res_err summed for all data samples. Thefollowing section presents the residual calculation for each gesturewith respect to each of the computed parameters.

We now detail how we determined which parameterization model for thepredictor bin would best differentiate gestures. A data set of positionand velocities of gestures is required to test each model. Using avision system, data was recorded for a slow, medium, and fast circulargesture. The data is the x and y position and velocity measurements fromthe image plane of the vision system, although for these simulationsonly one of the dimensions is used. There is a small transition timewhen a human begins a gesture. This transient is usually less than asecond long, but the residual error measurement has no meaning duringthis time. Therefore, gestures that last at least five seconds are used.The data recorded from a human gesture is termed “real gesture data.”

The total residual error was calculated by subjecting each predictor binto each gesture type. A measure of a model's usefulness is determined byexamining the ratio of the lowest residual error to the next lowestresidual error in each column. The worst “residual error ratio” is thesmallest ratio from all the columns because it is easier to classify agesture when the ratio is large.

The residual error results of the Linear with Offset Component are shownin Table 1. The residual errors for the slow and medium gestures, withrespect to their associated bins, are an order of magnitude lower thanthe other errors in their columns. The residual error of the fastgesture, with respect to the fast gesture bin, is one-forth the size ofthe closest residual error in its column (the medium gesture bin).Therefore, the Linear with Offset Component system is a good candidatefor a gesture model.

As seen in Table 2, the Van der Pol model is only a fair candidate forgesture discrimination. The residual error of the medium gesture withrespect to its gesture bin is only two-fifths smaller than the residualerror with respect to the slow gesture bin. Also, the residual errors inthe slow gesture column are not an order of magnitude apart.

The Van der Pol with Offset Component model is better at discriminatinggestures than the model without the offset term (see Table 3). Theresidual errors in the medium gesture's column are now an order ofmagnitude apart. Although the residual errors in the fast gesture'scolumn are not, the discrimination is still slightly better than in theLinear with Offset Component model.

Table 4 shows the residual errors associated with the Higher Ordermodel. This model is an improvement over the Van der Pol with OffsetComponent model, as the residual errors in the fast gesture's column arenow almost an order of magnitude apart.

Finally, Table 5 lists the residuals errors for the Velocity Dampingmodel. This is the best model for discriminating between gestures, asthe residual errors for each gesture with respect to their tuned binsare all at least an order of magnitude below the other residual errorsin their columns.

A comparison of the worst “residual error ratio” of each model weconsidered is summarized in FIG. 22, and suggests that the VelocityDamping model is the best choice for our application. However, thetechnique described here shows how more models could be derived andtested. For simple dynamic gesture applications, the Linear with OffsetComponent model would be used. For more complex gestures, a variation ofthe Velocity Damping model would be used.

Combining one-dimensional motions to form higher dimensional gestures.

We have shown how predictors can be used to recognize one-dimensionaloscillatory motions. Recognition of higher dimensional motions isachieved by independently recognizing multiple, simultaneously createdone-dimensional motions. For example, the combination of two oscillatoryline motions performed in perpendicular axis can give rise to circularplanar gestures, as shown in FIG. 23.

Humans have the ability to create these planar motions. However, theycan also make these motions in all three dimensions (for example,circles generated around different axis). To recognize these planargestures, performed in three-dimensional space, a vision system must beable to track a gesture's position through all three physicaldimensions. A binocular vision system has this capability, as does amonocular system with an attached laser range finder. Any of these suchvision systems can be used with our gesture recognition system toidentify three-dimensional gestures.

Development of a System to Recognize Static Gestures

Recognizing static hand gestures can be divided into localizing the handfrom the rest of the image, describing the hand, and identifying thatdescription. The module to recognize static hand gestures is to be bothaccurate and efficient. A time intensive process of evaluating handgestures would prevent the system from updating and following motionswhich occur in real time. The system is intended to interact with peopleat a natural pace. Another important consideration is that thebackground may be cluttered with irrelevant objects. The algorithmshould start at the hand and localize the hand from the surroundings.

Methodology

In order to meet these demands, the edges of the image are found with aSobel operator. This is a very fast linear operation which findsapproximations to the vertical and horizontal derivatives. In order touse only a single image, the greater of the horizontal and verticalcomponent is kept as the value for each pixel. Besides being quick tocalculate, an edge image avoids problems arising from attempting todefine a region by locating consistent intensity values or evenconsistent changes in intensity. These values can vary dramatically inone hand and can be very hard to distinguish from the background aswell.

In order to describe the hand, a box which tightly encloses the hand isfirst found. This allows a consistent description which is tolerant tochanges in scale. To locate this box, we assume a point within the handis given as a starting point. This is reasonable because the hand willbe the principal moving object in the scene. Moving objects may beeasily separated and the center of the largest moving area will be inthe hand. From this starting point, a prospective box edge is drawn. Ifthis box edge intersects an existing line, it must be expanded. Eachside is tested in a spiral pattern of increasing radius from the initialcenter point. Once three sides have ceased expanding the last side ishalted as well. Otherwise, the last side would often crawl up the lengthof the arm. The bounding box is shown in FIG. 24.

Once the hand has been isolated with a bounding box, the hand isdescribed (FIG. 25). This description is meant to be scale invariant asthe size of the hand can vary in each camera image. At regular intervalsalong each edge the distance from the bounding edge to the hand'soutline is measured. This provides a consistent description which may berapidly calculated. A description is a vector of the measured distances,allowing a very concise representation.

The last task of the static gesture recognition is to identify the newdescription. A simple nearest neighbor metric is used to choose anidentification. A file of recognized gestures is loaded in theinitialization of the program. This file consists of a list of namedgestures and their vector descriptions.

Considerations

The primary obstacle in static gesture recognition is locating andseparating the hand from the surroundings. Using sophisticated models ofthe hand or human body to identify with an image are computationallyexpensive. If orientation and scale are not very constrained, thiscannot be done in real time. Our system makes descriptions quickly andcan compare them to predefined models quickly.

The limitations of the current system are a result of being dependent onthe fast edge finding techniques. If lighting is highly directional,parts of the hand may be placed in shadow. This can cause odd, irregularlines to be found and defeat the normal description. If the backgroundimmediately surrounding the hand is cluttered with strongly contrastingareas, these unrelated lines may be grouped with the hand. This alsocauses unpredictable and unreliable descriptions. Such a background isvery difficult to separate without making assumptions about the handcolor or the size of the hand. An upper and lower bound are placed onthe size of the hand in the image, but these permit a wide range ofdistances to the camera and are needed to assure that enough of the handexists on image to make a reasonable description.

As long as the hand is within the size bounds (more than a speck ofthree pixels and less than the entire field of view) and the immediatesurroundings are fairly uniform, any hand gesture may be quickly andreliably recognized.

Multiple camera views can be used to further refine the identificationof static gestures. The best overall match from both views would be usedto define and identify the static gestures. Furthermore, the systemworks not just for “hand” gestures, but for any static type of gestures,including foot, limb, and full body gestures.

The Overall Gesture Recognition System

In this section, based on the discussed functional and representationalissues, we detail the specific components of our dynamic gesturerecognition system from an architectural and implementational viewpoint.Our system is composed of five modules: FIG. 4 illustrates the signalflow of the gestural recognition and control system, from gesturecreation, sensing, identification, and transformation into a systemresponse.

Gesture Creator

In the Gesture Creator module, a human or device creates a spatialmotion to be recognized by the sensor module. Our gesture recognitionsystem was designed to recognize consistent yet non-perfect motiongestures and non-moving static gestures. Therefore, a human as well as adevice can create the gestures which can be recognizable by the system.Human gestures are more difficult to recognize due to the wide range ofmotion that humans recognize as the same gesture. We designed ourgesture recognition system to recognize simple Lissagous gesture motions(repeating circles and lines), advanced motions such as “come here” and“go there”, and static hand symbols (such as “thumbs-up”).

Dynamic Gesture Lexicon

A gesture lexicon is a set of gestures used for communication or devicecontrol. We chose gestures for our lexicon based on the following:

-   -   Humans should be able to make the gestures easily.    -   Device gestures in the form of repeated motions should be        modeled the same as human gestures.    -   The gestures should be easily represented as a dynamic system.    -   The lexicon should match useful gestures found in real world        environments.

The dynamic gestures used in this system consist of threeone-dimensional oscillations, performed simultaneously in threedimensions (or two oscillations performed in two dimensions). A circleis such a motion, created by combining repeating motions in twodimensions that have the same magnitude and frequency of oscillation,but with the individual motions 90 degrees out of phase. A “diagonal”line is another such motion. To illustrate this, we define threedistinct circular gestures in terms of their frequency rates: slow,medium, and fast. Humans create gestures that we define as slow largecircles (slow), fast large circles (medium), and fast small circles(fast). More complex gestures can be generated and recognized, but thesesimple ones are used for illustrative purposes.

Main Three Gestures

Using the simpler Linear with Offset model (whose parameters are easierto understand than the more complex models), we represented a circle bytwo second order equations, one for each axis:

{dot over (x)} ₁ =x ₂

{dot over (x)} ₂=θ₁ x ₁+θ₂

and

{dot over (y)} ₁ =y ₂

{dot over (y)} ₂=θ₁ y ₁ +y ₂

Our gesture model has no “size” parameter. θ₁ is a frequency measure,and θ₂ is a drift component. The gestures were named “large”, “small”,“fast”, and “slow” due to the human motions used to determine theparameters (see FIG. 26). A fast small circle is used to represent afast oscillation because humans can not make fast oscillations usinglarge circles. Models with higher order terms would have parameters withdifferent representations.

Expanded Lexicon—Geometric Constraints

A total of 24 gestures are possible from this example representationwhen the following are distinct gestures: clockwise andcounter-clockwise circles, diagonal lines, one dimensional lines, andsmall and large circles and lines. Geometric constraints are required toexpand the lexicon, because different gestures can result in the sameparameters. FIG. 5 shows motions that would cause an identifier toproduce the same frequency measure and drift components as it wouldproduce when identifying a slow large circle. When x and y oscillatingmotions are 90 degrees out of phase, a clockwise circle is produced.Motions that are 270 degrees out of phase result in a counter clockwisecircle. In phase motions produce a line with a positive slope. When themotions are 180 degrees out of phase, a line with a negative slope isproduced. We can create additional gestures from the fast small circlein the same manner.

Given the various combinations of slow, fast, small, and large circles,the only one not used as a gesture is the slow small circle. Because,the slow small circle has the same oscillation frequency (medium) as thefast large circle, we need another geometric feature, the circle's size,to differentiate between these two gestures. As with the previousgestures, additional gestures can be created from these two gestures byvarying the phase relationships. FIG. 7 shows a representation of the 24gestures in this example lexicon.

Phase relationships are determined as follows. During the gesture, thex's and y's minimum and maximum image plane positions are computed. Ifthe x and y motions are out of phase, as in a circle, then when x or yis minimum or maximum, the other axis's velocity is large. Theclockwiseness of the motion is determined by looking at the sign of thisvelocity component. Similarly, if the x and y motion are in phase, thenat these extremum points both velocities are small. A similar method isused when the gesture is performed in three dimensions.

Sensor Module

Unmodified Cohu Solid State CCD cameras are used as the sensor devices.No filters were used and the background was not modified. The MatroxMeteor capture card allows us to scale a captured image to any sizewithout missing any frames. It will capture and transferfull-resolution, full-frame NTSC (640×480) or PAL (768×576) video inputin real-time (30 Hz).

The color tracking system (CTS) uses the color of the hand and itsmotion to localize the hand in the scene. The hardware of the CTS systemconsists of a color camera, a frame grabber, and an IBM-PC compatiblecomputer. The software consists of the image grabbing software and thetracking algorithm. Once the CTS is running, the graphical userinterface displays the live image from the color camera on the computermonitor. The operator can then use the mouse to click on the hand in theimage to select a target for tracking. The system will then keep trackof the moving target in the scene in real-time.

The color tracking system is developed on a BSD 4.0 UNIX operatingsystem. The hardware involved consists of a color camera, an imagecapture board and an IBM PC compatible. The software for the CTS iswritten in C and uses Motif for its graphical user interface (see FIG.27).

The present HTS system consists of a COHU 1322 color camera with aresolution of 494×768 pixels. The camera is connected to a Meteor imagecapturing board situated inside a Pentium-II 450 MHz IBM-PC compatiblecomputer. The Meteor board is capable of capturing color video images at30 frames per second. It is also able to capture these images at anyresolution below the resolution of the camera.

The graphical user interface for the CTS displays a live color imagefrom the camera on the computer screen. The user can then identify thetarget in the scene and click on it using the mouse. The CTS will thentrack the target in real-time. The flow chart of the tracking algorithmis shown in FIG. 28.

We capture the image using functions from the Meteor driver. To providereal-time operation, we setup the board to signal the program using asystem interrupt (SIGUSR2). Every time a new frame is ready, the Meteoralerts the program with an interrupt on this signal. The image capturefunction responds to the interrupt by transferring the current cameraimage to a buffer and processing it to find the target. The signalmechanism and its handling are what enable the system to operate inreal-time.

The graphical user interface of CTS displays the live camera image onthe screen. The user can start tracking by clicking the mouse on thetarget. This starts the tracking algorithm. The graphical user interfaceof the CTS is shown in FIG. 29.

Once the user clicks on the target in the image, we compute the averagecolor of a small region around this point in the image. This will be thecolor of the target region being tracked in the scene until it isreinitialized. Once tracking begins, we compute the position of thetarget region in the image using two methods. The first method tracksthe target when there is sufficient motion of the target in the image.The second method will take over when there is no motion of the targetin the scene.

Before choosing the methods for finding the target in the scene, thesystem checks for motion in a region near the current or estimatedtarget position using a motion detecting function. This functioncomputes the difference between the current image and the previousimage, which is stored in memory. If motion has occurred there will besufficient change in the intensities in the region. This will indicatemotion. The motion detection function will trigger if a sufficientnumber of pixels change intensity by a certain threshold value.

If the motion detection function detects motion, the next step is tolocate the target. This is done using the difference image and thetarget color. When an object moves between frames in a relativelystationary background, the color of the pixels changes between framesnear the target (unless the target and the background are of the samecolor). We compute the color change between frames for pixels near thetarget location. The pixels whose color changes beyond a threshold makeup the difference image. Note that the difference image will have areas,which are complementary. The pixels where the object used to be willcomplement those pixels where the object is at now. If we separate thesepixels using the color of the target, we can compute the new location ofthe target. The set of pixels in the difference image, which has thecolor of the target in the new image, will correspond to the leadingedge of the target in the new image. If we assume that the targetapproximates an ellipse of known dimensions, we can compute the positionof the center of the target (ellipse) from this difference image (seeFIG. 30).

The color of a pixel in a color image is determined by the values of theRed, Green and Blue bytes corresponding to the pixel in the imagebuffer. This color value will form a point in the three-dimensional RGBcolor space (see FIG. 31). For our tracking system, when we compute theaverage color of the target, we assume that the target is fairly evenlycolored and the illumination stays relatively the same. The averagecolor of the target is then the average RGB values of a sample set ofpixels constituting the target. When the target moves and theillumination changes the color of the target is likely to change. Thecolor matching function allows us to compute whether a pixel colormatches the target color within limits. When the illumination on thetarget changes, the intensity of the color will change. This will appearas a movement along the RGB color vector as shown in the figure below.In order to account for slight variations in the color, we further allowthe point in color space to lie within a small-truncated cone as shownin the figure. Two thresholds will decide the shape of the cone. One forthe angle of the cone and one for the minimum length of the colorvector. Thus, any pixel whose color lies within the truncated cone incolor space will be considered as having the same color as the target.

When the motion detection function fails to detect significant motion inthe scene, we use a static target matching function to compute itslocation. The function searches a small area about the current locationof the target to find the best fit in the image for the target. Thesearch will find the location of the target with the highest matchingvalue. We assume that the object is approximately elliptical. Theelliptical target is hypothesized at each point in the search space andthe matching metric is computed. This matching metric function uses acombination of edge and interior color matching algorithms to get asingle matching number.

The image capture board is capable of providing us with a 480×640-pixelcolor image at 30 frames per second. Processing such a large image willslow down the program. Fortunately, the nature of the tracking task issuch that, only a fraction of the image is of interest. This regioncalled the window of interest lies around the estimated position of thetarget in the new image. We can compute the location of the target inthe new image from the location of the target in the previous image andits velocity. This simple method is able to keep track of the targeteven when the target moves rapidly. We have found that the window ofinterest is typically 11100^(th) the area of the original image. Thisspeeds up the computation of the new target location considerably. Acomputer with a higher processing speed could process the entire imagewithout resorting to creating a region of interest.

Identification Module

The gesture recognition algorithms are located in the IdentificationModule. This module uses the position and velocity information providedby the sensor module to identify the gesture. The module is shown inFIG. 32 and consists of three components—the Dynamic Gesture Predictionmodule, the Static Gesture Identification module, and the OverallDetermination module (Which Gesture?). The output of the OverallDetermination module is sent to a screen display and to the SSM whichproduces an output based on the Gesture command received.

The Dynamic Gesture Prediction Module

The Dynamic Gesture Prediction module contains a bank of predictor bins(see FIG. 33). Each predictor bin contains a dynamic system model withparameters preset to a specific gesture. We assumed that the motions ofhuman circular gestures are decoupled in x and y. Therefore, there areseparate predictor bins for the x and y axes. In this example of threebasic two-dimensional gestures, a total of six predictor bins arerequired. The position and velocity information from the sensor moduleis fed directly into each bin.

The idea for seeding each bin with different parameters was inspired byNarendra and Balakrishnan's work on improving the transient response ofadaptive control system. In this work, they create a bank of indirectcontrollers which are tuned on line but whose identification models havedifferent initial estimates of the plant parameters. When the plant isidentified, the bin that best matches that identification supplies arequired control strategy for the system [Narendra 94].

Each bin's model, which has parameters that tune it to a specificgesture, is used to predict the future position and velocity of themotion. This prediction is made by feeding the current state of themotion into the gesture model. This prediction is compared to the nextposition and velocity, and a residual error is computed. The bin, foreach axis, with the least residual error is the best gesture match. Ifthe best gesture match is not below a predefined threshold (which is ameasure of how much variation from a specific gesture is allowed), thenthe result is ignored; no gesture is identified. Otherwise, geometricinformation is used to constrain the gesture further. A single gestureidentification number, which represents the combination of the best xbin, the best y bin, and the geometric information, is outputted to thetransformation module. This number (or NULL if no gesture is identified)is outputted immediately upon the initiation of the gesture and iscontinually updated.

The parameters used to initially seed each predictor bin were calculatedby feeding the data of each axis from the three example basic gesturesinto the recursive linear least squares. The values for each bin aresummarized in Table 6.

The Static Gesture Identification Module

The Static Gesture Identification module only searches for staticgestures when the hand motion is very slow (i.e. the norm of the x and yvelocities is below a threshold amount). When this happens, the modulecontinually identifies a static gesture or outputs that no gesture wasfound.

The static gestures may be easily expanded by writing new gesturedescriptions to a configuration file. Each gesture is described by aname tag, width, height, x location, y location, base side, and threevectors (in this example, each consisting of 15 integers) describing theprofile of the hand. Because profiles may be significantly different dueto varying tilts of the hand, multiple descriptions of fundamentally thesame gesture may be desired. The initial or last line may also be lessreliable due to missing the contours of the hand edge image.

The following are example parameter files (see Table 7). In each thename string is followed by an arm side, width, height, x location and ylocation. The arm parameter is simply an integer corresponding to above,below, right, or left. The width and height are measured in pixels. Thex and y location are 0 if the location is not important or +1 or −1 torestrict recognition of a gesture to one particular quadrant. Thefollowing three vectors are the extreme side (the end of the hand) thenthe top or left side followed by the bottom or right side. Thedetermination of which side is being represented is determined by thearm side parameter. For example, if the base side is from below (as inthe Halt gesture below) the first line is from above, then from theleft, then from the right. Right and left refer to the overall image—notthe facing of the imaged person.

Another method used for this part is to parameterize each part of thehand (palm, digits, and wrist) as a set of connected “blobs”, that is,three-dimensional shapes which are connected together geometrically. Asbefore, a configuration file would be used to defile how these blobs areconnected, with the vision system identifying the blobs which thismodule sticks together.

The Overall Determination Module

This “Which Gesture?” module takes input from both the Static andDynamic Gesture modules. When the velocity is small, then a staticgesture is observed. When the velocity is greater than a thresholdvalue, then a dynamic gesture is observed. The gesture identified iscontinuously outputted, and can therefore change value over time (theoutput can even be that no gesture was identified). The gestureidentified is sent to the transformation module.

Transformation Module

The transformation module take a gesture type as its input anddetermines what to do with it. In the case of this system, the gestureis converted to parameters which represent the static or dynamicgesture, which is sent to the system which uses this information toproduce a response.

System Response

The gesture command can be used for a wide variety of purposes. Theseinclude:

-   -   Commands into a virtual reality simulator, to control and        interact with the environment.    -   Commands for a self service machine (SSM), such as a public        information kiosk or Automated Teller Machines.    -   Commands to control an actuated mechanism, such as a robot arm        or mobile robot.    -   Commands to control any device (such as a home appliance).

It is important to note that these devices can be controlled usingstatic gestures, dynamic gestures, or a combination of the two. Thus,there is more information available to these system from the gestureinput device, thereby allowing for a greater ability for humans tocommand and control them.

The key features of our architecture are the prediction modules and thesignal flow from gesture creation to system response. The other modulescould be replaced with functionally equivalent systems without changingthe structure of our architecture. For example, instead of a human, arobot could create the gesture. Alternatively, one could create thegesture using a stylus, with a graphics tablet replacing the visionsystem in sensor module S. The graphics tablet would output the x and ycoordinates to the identification module I. Similarly, module R could bea robot, one as complex as a six degree of freedom robot arm or assimple as a stepper motor based camera platform. The former mechanismrequires a more complex transformation scheme in module T, while thelatter system needs only a simple high level command generator.

As discussed earlier, the static and dynamic identification modulescontains the majority of the required processing. Compared to most ofthe systems developed for gesture recognition (for example, see [Darrell93] and [Murakami 91], for more details, see [Cohen 96]), this systemrequires relatively little processing time and memory to identify onegesture feature. This makes it possible to create a system with theability to identify multiple features in parallel. A sophisticatedmodule could then examine the parallel gesture features and infer somehigher level motion or command.

The Behavior Recognition System

Just as the gesture recognition module is built on a bank of predictorbins, the behavior recognition system is composed of a bank of gesturerecognition modules. Each module focuses on a specific point of the body(such as a foot or knee). As that point moves through space, a “gesture”is generated and identified. The combination of gestures from thosepoints are what we define as a motion behavior, which can be categorizedand identified. The system, illustrated in FIG. 34, details the behaviorrecognition system. The simplicity of the behavior recognition system ispossible because of the strength and utility of the gesture recognitionmodules.

Overall System Flow

The signal flow proceeds as follows. A user is tagged at various bodylocations automatically by the vision system. The data is acquired at 30Hz and sent to a parser which splits off the data from each specificbody location to its own gesture recognition module (GRM). There is oneGRM for each tagged feature. Each GRM outputs which gesture itrecognized (if any) which is again sent to an identification module. Theidentification module matches the gestures to their body location,defining a behavior. If this behavior matches one from a set ofpredefined behaviors, then this information is outputted.

The Parser

In the Parser module, the data, which is input as a stream ofconsecutive x,y,z,time coordinates from each tagged body location, issplit up according to body location and sent to an appropriate GMI. Thismodule needs to be changed whenever the input data is of a differentformat. Runtime variables define how many body parts are being tracked,and therefore the parser uses this information to determine the numberof GMI bins and how to split up the data properly.

Gesture Recognition Modules (GRMs)

The time and coordinate data from each body feature is used as inputs toan appropriate GRM. Each GRM module is exactly as described in sectionII a, except that these modules handle three dimensional points insteadof just two.

Behavior Identification Module

The Behavior Identification Module accepts as inputs gesture types andbody locations from the GRMs. Various combinations of gestures atspecific body locations are designated as behaviors, and if a match isfound, then the program outputs that match.

Behavior Recognition Experiments and Data

We performed experiments to test the behavior recognition system. First,a variety of behaviors were performed and served as the baseline foridentification. Then these behaviors were repeated and the data sentthrough the system.

The behaviors centered on repeated leg and waist motion. The three maintypes of behaviors performed were:

Jumping Jacks: Start by standing straight, legs together, then move thefeet out sideways and back together.

Walking in place: Move the feet in a slow walking motion.

Jogging in place: Move the feet up and down in an exaggerated quickmotion.

Other behaviors used for experimentation, some of which included fullthree-dimensional motion:

Squats: Bend the knees to lower the body, then rise again.

Walking: Walk slowly towards the sensors.

Running: Run towards the sensors.

Skipping: Skip towards the sensors.

Hopscotch: Jump and land on one foot, then jump and land on both feetspread apart.

Rigorous experiments were performed using the first three behaviors. Weused the Velocity Damping Terms gesture model to capture the richness ofthe presented motions. There was clear discrimination between themotions identified by the various identification bins, and, as shownbelow, behavior recognition was possible.

For the Jumping Jacks behavior, the eight theta's for each axis for eachsensor are:

Sensor 1:

x-axis: 26.8199, 57.4879, 0, 0, 0, 1751.95, 0, 0,y-axis: 0, 0, −257.759, 0, 0, 15.9921, 58.561, 0,z-axis: 0, 0, 24.4205, 57.9981, 0, 0, 0, −1026.36,

Sensor 2:

x-axis: 17.8334, 58.4356, 0, 0, 0, 1691.1, 0, 0,y-axis: 0, 0, −35.2513, 0, 0, 6.14527, 59.7759, 0,z-axis: 0, 0, 28.6432, 57.4095, 0, 0, 0, 918.332,

Sensor 3:

x-axis: 15.0551, 58.7612, 0, 0, 0, 1186.79, 0, 0,y-axis: 0, 0, −476.275, 0, 0, 7.1385, 59.4896, 0,z-axis: 0, 0, 6.74113, 59.9307, 0, 0, 0, −544.907

Sensor 4:

x-axis: 10.8193, 58.9695, 0, 0, 0, −1210.42, 0, 0,y-axis: 0, 0, 341.434, 0, 0, 9.92934, 59.1288, 0,z-axis: 0, 0, 24.3505, 58.0358, 0, 0, 0, −142.869

Sensor 5:

x-axis: 16.7313, 58.3415, 0, 0, 0, 4060.06, 0, 0,y-axis: 0, 0, 819.198, 0, 0, 15.3747, 58.6259, 0,z-axis: 0, 0, 27.1073, 57.975, 0, 0, 0, 612.659.

The residual results are shown in Table 8. As anticipated, the bolddiagonal cells have lower residuals than all the other cells in theirrespective rows for each gesture in the behavior, demonstrating that oursystem can discriminate this behavior type. Some of the residuals in abin are very close in value to others sensors. This is because they are,in fact, the same gesture. For example, sensor 4 and sensor 5 are placedon the knee and foot of the same leg, which would make the sameoscillatory motion during a behavior, so of course the residual valueswould be similar.

For the Walking behavior, the eight theta's for each axis for eachsensor are:

Sensor 1:

x-axis: 24.304, 57.4632, 0, 0, 0, −351.126, 0, 0,y-axis: 0, 0, −168.974. 0, 0, 23.2088, 57.6762, 0,z-axis: 0, 0, 22.6047, 57.7623, 0, 0, 0, 1150.72,

Sensor 2:

x-axis: 19.8496, 57.8372, 0, 0, 0, −1017.67, 0, 0,y-axis: 0, 0, 31.8642, 0, 0, 26.8075, 57.5024, 0,z-axis: 0, 0, 25.6103, 57.9468, 0, 0.000123365, 0, −358.633,

Sensor 3:

x-axis: 10.2042, 59.182, 0, 0, 0, −617.508, 0, 0,y-axis: 0, 0, 498.471, 0, 0, 30.6624, 56.95, 0,z-axis: 0, 0, 22.534, 57.7824, 0, 0, 0, 598.156,

Sensor 4:

x-axis: 24.6263, 57.6805, 0, 0, 0, −73.7837, 0, 0,y-axis: −0.000250812, 0, 125.269, 0, 0, 19.2459, 58.205, 0,z-axis: 0, 0, 18.6133, 58.2271, 0, 0, 0, −195.928,

Sensor 5:

x-axis: 26.3399, 57.6672, 0, 0, 0, −522.552, 0, 0,y-axis: −0.000136806, 0, 115.618. 0, 0, 18.8083, 58.326, 0,z-axis: 0, 0, 19.0266, 58.1954, 0, 0, 0, 474.65,

The residual results are shown below in Table 9. Again, the bolddiagonal cells have lower residuals than all the other cells in theirrespective rows for each gesture in the behavior, demonstrating that oursystem can discriminate this behavior type.

For the Running behavior, the eight theta's for each axis for eachsensor are:

Sensor 1:

x-axis: 22.9112, 57.8263, 0, 0, 0, −1763.57, 0, 0,y-axis: 0, 0, −489.467, 0, 0, 11.7958, 58.779, 0,z-axis: 0, 0, −3.51229, 61.0138, 0, 0, 0, 713.328,sensor 2:x-axis: −2.11517, 60.7157, 0, 0, 0, −40235.2, 0, 0,y-axis: 0, 0, −4506, 0, 0, −79.0879, 70.5397, 0,z-axis: 0, 0, −78.8084, 70.6087, 0, 0, 0, −375964,

Sensor 3:

x-axis: 24.5412, 57.6338, 0, 0, 0, −2805.13, 0, 0,y-axis: 0, 0, −211.096, 0, 0, 23.1457, 57.6718, 0,z-axis: 0, 0, 20.9598, 58.3911, 0, 0, 0, 773.77

Sensor 4:

x-axis: 20.1377, 58.218, 0, 0, 0, 4557.85, 0, 0,y-axis: 0, 0, 607.713, 0, 0, 11.9292, 59.0339, 0,z-axis: 0, 0, 16.2398, 58.6524, 0, 0, 0, −2667.72,

Sensor 5:

x-axis: 29.6411, 56.9948, 0, 0, 0, 1093.19, 0, 0,y-axis: 0, 0, 954.695, 0, 0, 14.4107, 58.6439, 0,z-axis: 0, 0, 20.9606, 58.0327, 0, 0, 0, 3497.27.

The residual results are shown below in Table 10. As before, the bolddiagonal cells have lower residuals than all the other cells in theirrespective rows for each gesture in the behavior, demonstrating that oursystem can discriminate this behavior type. As for quadruped locomotion,Kelso [Kelso 95] hints that there is dynamic mechanical abstractionwhich can be studied to explain certain features of quadrupedlocomotion, specifically the limb frequencies of animals moving aboutthe Serengeti plains. As he states: “When plotted against limb length ormass, the stepping frequency, from Thompson's gazelle to the blackrhinoceros, falls on three straight lines, one for each locomotorymode.” Thus, it should be apparent to one of skill in the art that theinvention is applicable to quadruped locomotion as well throughappropriate extension.

APPLICATION AREAS

Although one application of the invention is threat assessment, otheruses are possible, including the generalized discrimination ofsuspicious or otherwise curious behaviors from normal activities. Thesystem will not only perform surveillance activities, but make criticalvisual surveillance observations of locations for which there are notenough personnel to cover, or which are just simply too dangerous. Suchsurveillance tasks may include:

-   -   Classifying: identify objects or classes of objects.    -   Tracking: identify moving objects or people, and predict their        future behavior.    -   Patrolling: determine when something new enters or leaves the        field of view, then track and classify it.    -   Warning: determine threats in the area that need to be signaled        back to an appropriate area. Such situations include, but are        not limited to, vehicles speeding towards a facility, people        loitering, climbing perimeter walls, carrying weapons, and        transferring objects.

In the commercial area, this system provides the foundation for a largenumber of gesture recognition and behavior recognition applications.Following is a list of example commercial applications to which thetechnology is applicable:

ATM Control. Given our understanding of the gesture recognition, thegesture recognition system could be used for the control of AutomaticTeller Machines (ATMs), as well as any other self-service machine orkiosk.

Crime Prevention. As discussed earlier, combinations of body gesturescan be viewed as motion gaits, which can in turn be possibly identifiedas certain physical activities. Therefore, the same camera that isdetecting gestures for the ATM (and any other self-service machine orkiosk) control can be used to identify the physical activity in thearea. Such activities could include possible robberies or assaults.Having such crime detection features in place would greatly increase thesafety of using an ATM.

General User Interfaces. This gesture and behavior recognition systemcan be incorporated into general user interfaces, to augment other inputdevices such as the mouse, keyboard, and voice commands. A system whichcould recognize behaviors would be useful in training people to use suchdevices.

As in the commercial arena, the gesture and behavior recognition systemwill have many uses in the military arena. The following is a list ofmilitary applications which we can expect to pursue in future projects.

Enhanced User Interfaces. As in the commercial application, the gestureand behavior recognition system can be incorporated into militarycomputer systems that would benefit from an additional gestural userinterface.

Surveillance. Since the dynamic gesture recognition can be used todetect physical activity at an ATM, it can also be used to detectactivities in a combat environment. These activities not only includerunning, walking, and firing a weapon, but would also include thedetection of group behaviors and formations.

We claim:
 1. A method of controlling a device without touching it,comprising the steps of: storing a dynamic motion model includinginformation describing a particular dynamic gesture to be recognized inthe form {dot over (x)}=f (x, θ) where {dot over (x)} is a vectordescribing the position and velocity components of the gesture and θ isa tunable parameter; sensing a human motion to be recognized; extractingposition and velocity components of the sensed motion; determining ifthe motion is a recognizable gesture based upon the extracted positionand velocity components and tunable parameters of the sensed motion; andif the motion represents a recognizable gesture, generating a command tocontrol a device in accordance with the recognized gesture.
 2. Themethod of claim 1, wherein the step of sensing includes anelectromechanical process using an electrogoniometer orelectrogoniometric system.
 3. The method of claim 2, wherein theelectromechanical process uses passive reflective or activelyilluminated markers to calculate positions or velocities.
 4. The methodof claim 2, wherein the electromechanical process uses force datagathered from a force plate or force dynamometer.
 5. The method of claim2, wherein the electromechanical process uses strain gauge orpiezoelectronic transducers.
 6. The method of claim 1, including thestep of classifying the gesture as a static gesture if the slowness ofthe motion falls below a predetermined threshold.
 7. The method of claim1, wherein the device is a virtual-reality simulator or game.
 8. Themethod of claim 1, wherein the device is a self-service machine.
 9. Themethod of claim 1, wherein the device forms part of a robot.
 10. Themethod of claim 1, wherein the device forms part of a commercialappliance.
 11. The method of claim 1, wherein the step of sensingincludes the use of a computer vision system.
 12. A gesture-controlleddevice, comprising: a sensor for capturing a human gesture to berecognized; a processor executing a feature tracking algorithm thatoutputs position and velocity components associated with the capturedgesture; a gesture identification module that converts the position andvelocity components to into parameters representing a static or dynamicgesture; wherein the identification of a dynamic gesture is based on astored dynamic motion model including information describing aparticular dynamic gesture to be recognized in the form {dot over(x)}=f(x, θ) where {dot over (x)} is vector describing the position andvelocity components of the gesture and θ is a tunable parameter; andcontrol apparatus that controls the operation of the device inaccordance with the static or dynamic gesture.
 13. Thegesture-controlled device of claim 12, wherein the sensor is anelectromechanical device.
 14. The gesture-controlled device of claim 12,wherein the sensor uses passive reflective or actively illuminatedmarkers to calculate positions or velocities.
 15. The gesture-controlleddevice of claim 12, wherein the gesture identification module isoperative to classify the gesture as a static gesture if the slowness ofthe motion falls below a predetermined threshold.
 16. Thegesture-controlled device of claim 12, wherein the device is avirtual-reality simulator or game.
 17. The gesture-controlled device ofclaim 12, wherein the device is a self-service machine or robot.
 18. Thegesture-controlled device of claim 12, wherein the device is acommercial appliance.
 19. The gesture-controlled device of claim 12,wherein the sensor is an image sensor.